Examples of emerging two-way HFC systems are addressed in the following references: IEEE Project 802 14/a Draft 3 Revision 1, xe2x80x9cCable-TV Access Method and Physical Layer Specification,xe2x80x9d IEEE, Apr. 13, 1998; MCNS Interim Specification, xe2x80x9cData Over Cable Interface Specificationsxe2x80x94Radio Frequency Interface Specification,xe2x80x9d MCNS Holdings, L. P., Mar. 26, 1997; C. A Eldering, N. Himayat, and F. M. Gardner, xe2x80x9cCATV Return Path Characterization for Reliable Communications,xe2x80x9d IEEE Commun. Mag., vol. 33, pp. 62-69, August 1995; C. Bisdikian, K. Maruyama, D. I. Seidman, and D. N. Serpanos, xe2x80x9cCable Access Beyond the Hype: On Residential Broadband Data Services over HFC Networks,xe2x80x9d IEEE Commun. Mag., vol. 34, pp. 128-135, November 1996.
Sections 4 and 6 of the xe2x80x9cMCNS Interim Specification, xe2x80x98Data Over Cable Interface Specificationsxe2x80x94Radio Frequency Interface Specification,xe2x80x99xe2x80x9d reference are incorporated by reference. The multichannel multipoint distribution service (MMDS) and the local multipoint distribution service (LMDS) are exemplified in a reference by W. Honcharenko, J. P. Kruys, D. Y. Lee, N. J. Shah, xe2x80x9cBroadband wireless access,xe2x80x9d IEEE Commun. Mag., vol. 35, pp. 20-27, January 1997.
The design of a HC modem transmitter and station modem receivers for downstream transmission presents some technical challenges due to transmission rates in the order of 30 to 45 Mbit/s per downstream channel. However, owing to the continuous broadcast mode of downstream transmission over a channel with low distortion and high signal-to-noise ratio (typically 42 dB by regulation) well known signal-processing techniques can be applied. In the upstream direction, implementation of physical (PHY) layer transmission and MAC layer functions pose considerable technical challenges. First, because signals may be transmitted in bursts, HC receivers with fast synchronization capabilities are essential. Second, individual station signals must be received at the HC at defined arrival times and power levels. Therefore, determination of the round-trip delay between the HC and each individual station and individual transmit power control for each station to compensate for widely varying attenuations in the upstream direction are important functions. Third, the upstream channel is generally much more noisy and subject to more distortion than the downstream channel.
Transmission schemes based on single-carrier quadrature-amplitude modulation (QAM) can represent a robust solution for upstream transmission. These schemes, however, are generally less efficient and robust in the presence of impulse noise and narrow-band interference than multicarrier modulation techniques, also known as orthogonal frequency division multiplexing (OFDM), addressed in J. A. C. Bingham, xe2x80x9cMulticarrier Modulation for Data Transmission: An Idea Whose Time Has Come,xe2x80x9d IEEE Commun. Mag., vol. 28, pp. 5-14, May 1990.
The Bingham, et al. reference is incorporated by reference in its entirety. In general, OFDM systems employ M orthogonal subcarriers for parallel transmission of blocks of M symbols over M subchannels. For example, versions of orthogonal frequency division multiplexing (OFDM) known as discrete multitone (DMT) modulation and discrete wavelet multitone (DWMT) modulation are considered as transmission schemes for the digital subscriber line. For details see references by J. S. Chow, J. C. Tu, and J. M. Cioffi, xe2x80x9cA Discrete Multitone Transceiver System for HDSL Applications,xe2x80x9d IEEE J. Select. Areas Commun., vol. 9, pp. 895-908, August 1991; xe2x80x9cNetwork and Customer Installation Interfacesxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface,xe2x80x9d ANSI TIE1.413-1995, August 1995; and S. D. Sandberg and M. A. Tzannes, xe2x80x9cOverlapped Discrete Multitone Modulation for High Speed Copper Wire Communications,xe2x80x9d IEEE J. Select. Areas Commun., vol. 13, pp. 1571-1585, December 1995.G., all of which are incorporated by reference in their entireties.
Multitone modulation with a high level of subchannel spectral containment was mentioned in both European Patent application bearing the title xe2x80x9cCommunications Network Systemxe2x80x9d, G. Cherubini and G. Ungerboeck, filed on the same day as the present application and currently assigned to the assignee of the present application, and J. Karaoguz, J. Yu, V. Eyuboglu, xe2x80x9cComparison of Single-Carrier, Multi-Carrier, and Spread Spectrum Modulation for Upstream PHY Layer in HFC CATV Networks,xe2x80x9d IEEE 802.14 contribution 98-018, July 1998. These references are incorporated by reference herein in their entireties.
In the Karaoguz et al. reference, the filters are ideal unrealizable square-root raised cosine filters with zero roll-off. As mentioned above, a multiple access scheme for upstream transmission in HFC networks would have to be suitable for transmission of short messages in burst mode, and also would have to ensure reliable communications in the presence of impulse noise and narrowband interference. To accomodate these requirements, we propose a hybrid time-division multiple access (TDMA)/synchronous code-division multiple access (S-CDMA) scheme based on FMT modulation.
The present invention relates to multiple-access communications network systems in which a head-end controller (HC) broadcasts data and medium-access control (MAC) information over a set of downstream channels to several stations, and these stations send information to the HC over a shared upstream channel. Illustrative examples of systems exhibiting these characteristics arc two-way hybrid fiber/cable (HFC) systems and their wireless counterparts, i.e., multichannel multipoint distribution service (MMDS) and local multipoint distribution service (LMDS).
In accordance with the present invention, there is an upstream transmission in HFC networks, a new transmission technique related to OFDM, named filtered multitone (FMT) modulation. This is advantageous because FMT modulation exhibits significantly lower spectral overlapping between adjacent subchannels and provides higher transmission efficiency than DMT, and is better suited for passband transmission than DWMT.
A digital transmission scheme is disclosed for use in a multiple access communications network system with a system comprising an upstream channel with a bandwidth of 1/Tc Hz for transmission of information from remote stations to a head-end node, where a baseband signal is transmitted as passband signal employing a modulator, said baseband signal being generated by employing M subcarriers for transmission over M subchannels of blocks of M symbols, each taken from a symbol constellation, where the transmission over the i-th subchannel, with i=0, . . . , Mxe2x88x921, takes place at a modulation rate of 1/(mi Tc) symbols/s, the i-th subchannel has a bandwidth of 1/(miTc) Hz, where the numbers mi satisfy the condition:             ∑              i        =        0                    M        -        1              ⁢          1              m        i              =  1
and, at least one subchannel has spectral nulls at the bandedges, and at least one subchannel has zero guard bands.